1. Field of the Invention
The present invention relates to a spark plug having a metallic shell that is crimped so as to integrally fix an insulator thereto.
2. Description of the Related Art
Conventionally, a spark plug is used for ignition of an internal combustion engine. A spark plug typically includes a metallic shell holding an insulator into which a center electrode is inserted, and a ground electrode welded to a front end portion of the metallic shell. The distal end of the ground electrode faces the front end of the center electrode, thereby forming a spark discharge gap therebetween. Spark discharge occurs between the center electrode and the ground electrode. In such a spark plug, in which a step portion formed on an outer circumferential surface of the insulator is supported by a step portion formed on a front-end-side inner circumferential surface of the metallic shell, the insulator is crimped by a crimp portion provided at the rear end of the metallic shell. Thus, the insulator and the metallic shell are fixed together, while close contact between the two steps is maintained. Further, talc and/or a packing may be accommodated within the interior of the crimp portion, so that the insulator and the metallic shell are fixed more reliably, and air-tightness is secured.
In recent years, with increasing demand for enhanced power output of automotive engines and reduced fuel consumption, there is a demand for a reduction in size and diameter of a spark plug from the viewpoint of securing freedom in engine design. One conceivable solution for reducing the size and diameter is to reduce the respective sizes of the spark plug components. For example, the size and diameter of the insulator can be reduced. However, if the diameter of the entire insulator, which is formed of a fired ceramic, is reduced, the risk of breaking the insulator increases due to a reduction in strength. Therefore, reducing the diameter of the insulator is not a preferred approach. In view of the above, attempts have been made to reduce the overall size and diameter of a spark plug by reducing the diameter of the metallic shell which is of higher strength.
Reducing the diameter of a spark plug in this way requires a reduction in the wall thickness of the metallic shell or a reduction in the clearance between the insulator and the metallic shell. As an example structure for reducing the clearance, the diameter of an intermediate trunk portion of the insulator which is used to hold the insulator within the metallic shell may be reduced so as to be close to that of a rear trunk portion formed on a rear end side of the intermediate trunk portion. Since this intermediate trunk portion includes a portion which has the largest outer diameter (a maximum diameter portion), if the diameter of the metallic shell is reduced to match the reduced outer diameter of the intermediate trunk portion, the diameter of the entire spark plug can be reduced. However, since the crimp portion comes closer to the rear trunk portion, it becomes difficult to pack talc or the like into the interior of the crimp portion (the clearance between the crimp portion and the rear trunk portion) as in the case of the above-described conventional structure. In such a case, hot crimping is preferably performed so as to maintain air-tightness after crimping (see, for example, Patent Document 1). Specifically, a thin wall portion provided on a trunk portion of the metallic shell is heated so as to reduce resistance to deformation, and the crimp portion is crimped in this state. As a result, crimping by means of plastic deformation of the crimp portion and crimping by making use of a difference in thermal expansion between the insulator and the metallic shell are realized simultaneously. In this manner, a shoulder portion of the intermediate trunk portion of the insulator is pressed toward the front end by means of the crimp portion. Thus, air-tightness can be secured between the step portion of the metallic shell and the step portion of the insulator without packing talc or the like.
Incidentally, for the purpose of, for example, preventing flashover, a glaze layer is formed on a portion (rear trunk portion) of the insulator, which portion is exposed from the rear end portion of the metallic shell. As has been empirically known, the breakage resistance of the insulator can be improved when the glaze layer is formed to extend from the rear end of the insulator, covering the entire rear trunk portion, and further covering the shoulder portion of the intermediate trunk portion. Therefore, it is desirable to reliably form the glaze layer in the above-described portion of the insulator of the spark plug.
In general, the glaze layer is formed as follows. A glaze slurry to be applied to an insulator is prepared by crushing a glass component which constitutes the glaze layer and mixing it into a solvent medium. By use of a roller, a sprayer, or the like, this glaze slurry is applied to a predetermined portion of a horizontally supported insulator; that is, a region extending from the rear end of the insulator to the shoulder portion of the intermediate trunk portion thereof. Subsequently, the insulator is dried in order to improve workability. Subsequently, the insulator applied with the glaze slurry is placed in a heating furnace, and is fired at a predetermined temperature, whereby a glaze layer is formed (hereinafter, this step is also referred to as “glaze firing”).
In the above-described glaze firing, when firing is performed with the insulator held horizontally, in some cases, the heated and softened glaze flows downward and forms a biased layer. If a formed glazed layer has a non-circular cross section, flashover disadvantageously becomes difficult to prevent, and appearance is impaired. A conceivable measure for avoiding this problem is to fire the insulator while rotating the same. Alternatively, firing can be performed with an insulator held vertically, which is more efficient since rotating the insulator becomes unnecessary. Moreover, in view of the above-described problems, firing is desirably performed with the rear end of an insulator directed upward.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2003-257583